The present invention relates to a method of producing a semiconductor device, and more particularly, to a method of producing a semiconductor device in which a crystalline silicon film obtained by crystallizing an amorphous silicon film serves as an active region. In particular, the present invention is effective for a semiconductor device using thin film transistors (TFTs) formed on a substrate with an insulating surface, and can be utilized for an active matrix type liquid crystal display device, a contact type image sensor, a three-dimensional IC, and the like.
Recently, aiming to realize a large-size, high-resolution liquid crystal display, a high-speed, high-resolution contact type image sensor, a three-dimensional IC and the like, there have been trials to form a high-performance semiconductor device on an insulating substrate such as glass, or on an insulating film. In general, thin film-shaped silicon semiconductors are used for semiconductor devices to be used for these devices. The thin film-shaped silicon semiconductors are roughly classified into two types, i.e., those semiconductors made of amorphous silicon (a-Si) semiconductors and those semiconductors made of crystalline silicon semiconductors.
Amorphous silicon semiconductors, of which fabrication temperatures are low, can be easily fabricated by a vapor phase method, are superior in mass productivity, and thus they have been used most commonly. However, the amorphous semiconductors are inferior in physical properties such as electrical conductivity to silicon semiconductors having crystallinity. Therefore, the establishment of the fabrication method of semiconductor devices made of silicon semiconductors having crystallinity has been strongly desired in order to obtain improved high-speed properties. As the silicon semiconductors having crystallinity, polycrystalline silicon, microcrystalline silicon and the like have been known.
As the method of obtaining the thin film silicon semiconductors having crystallinity, the following methods (1)-(3) have been known:                (1) A method comprising directly forming a film having crystallinity at the time of film formation;        (2) A method comprising forming an amorphous semiconductor film and causing it to have crystallinity using the energy of laser light; and        (3) A method comprising forming an amorphous semiconductor film and causing it to have crystallinity by applying thermal energy.        
However, in the method (1), the crystallization proceeds simultaneously with the film formation step, and thus it is essential to thicken the silicon film in order to obtain crystalline silicon with a large grain size. Accordingly, it is technically difficult to form a film having equally good physical properties of semiconductors on the whole surface of the substrate.
Further, in the method (2), since the crystallization phenomenon in the process of fusion to solidification is utilized, grain boundaries are treated appropriately a high-quality crystalline silicon film is obtained although the grain size of crystal grains is small. Taking as an example an excimer laser, which is most commonly used, no excimer lasers having sufficient stability have been obtained yet. Therefore, it is difficult to treat the whole surface of the large-area substrate uniformly, and a further improvement in the aspect of hardware is desired.
Further, the method (3) is advantageous over the above methods (1) and (2) in regard to the uniformity and stability inside the substrate. However, heat treatment extending for a long period of time, e.g., about 30 hours, at 600° C. is required. Therefore, there are such problems as long treatment time and low throughput. Further, as the means of improving crystallinity, a technique of further conducting heat treatment at a high temperature of about 1000° C. in an oxygen atmosphere has been utilized. However, an inexpensive glass substrate cannot be used in this process. Furthermore, regarding the device properties, only low properties such as a field-effect electron mobility of about 100 cm2/Vs have been achieved in TFTs.
As a countermeasure against these methods, a method in which the method (3) is improved to obtain a high-quality crystalline silicon film is proposed in JP-A6-244103. This method is intended to achieve the reduction of heating temperature, the shortening of treatment time, and the improvement of crystallinity by using metal elements promoting the crystallinity of the amorphous silicon film. Specifically, a minute amount of a metal element or elements such as nickel, palladium, etc. is introduced to the surface of the amorphous silicon film and then heating is performed.
The mechanism of crystallization at a low temperature is understood as follows. The nucleation in which a metal element acts as crystal nuclei takes place at an early stage, and then the metal element serves as a catalyst to promote crystal growth, resulting in rapid proceeding of crystallization. In this sense, such a metal element is hereinafter referred to as a “catalyst element”. In a crystalline silicon film that has grown with the promotion of crystallization by the catalyst element or elements, the inside of one grain thereof is constituted of a network of a number of columnar crystals, while the inside of one grain of a crystalline silicon film crystallized by the conventional solid phase growth method has a twin crystal structure. The inside of each columnar crystal is in an almost ideal single crystal state.
Further, according to JP-A-7-221017, a catalyst element is introduced into an amorphous silicon film and then heat treatment is performed for a short period of time to form crystal nuclei only. After that, the irradiation with laser light is performed to cause crystallization.
Although a silicon film crystallized using a catalyst element has good crystallinity, there are many defects in each of the crystal grains. Accordingly, as the silicon film to be used for the active layer of high-performance semiconductor devices, high-quality crystalline silicon films with fewer defects are desired. In order to improve crystallinity more, there are a method in which, after crystallization using a catalyst element, heat treatment is performed at a higher temperature (800-1100° C.) in an oxidative atmosphere, and a method in which, after crystallization using a catalyst element, the irradiation with laser light is performed. The former method (high-temperature oxidation thermal treatment) is what is called a high-temperature process, and thus an inexpensive glass substrate cannot be used.
Accordingly, premising the use of an inexpensive glass substrate, the latter method (laser irradiation) is to be used. In the crystalline silicon film obtained by introducing a catalyst element and performing a heat treatment, each crystal grain is constituted of a network of columnar crystals, each having a width of 800-1000 Å. Although the inside of the individual columnar crystals is in a single crystal state, many crystal defects such as dislocation are present inside the crystal grains due to the bend or branching of these columnar crystals. Laser irradiation is intended to make defects inside the crystal grains disappear from columnar crystal components having good crystallinity. In reality, however, it is very difficult to realize that.
In fact, when a crystalline silicon film crystallized by the promotion of catalyst elements is irradiated with laser light, there is almost no effect at a low laser power. The laser irradiation at a low laser power only retains the original crystalline state without any great improvements. On the other hand, laser irradiation at a high power resets the original crystalline state to a state similar to a state crystallized only by laser light. It is extremely difficult to form a crystalline state that is in the middle of the state obtained by low laser power and the state obtained by high laser power, and there is no margin of laser power at all.
As a result, it is difficult to improve crystallinity for formation of the active regions of TFTs by irradiating the crystalline silicon film, which has been crystallized with the aid of the catalyst element or elements, with laser light. Specifically, even if the irradiation of laser light is performed, the resulting TFTs may exhibit such properties as low electric current-driving ability that has almost no big difference from the electric current-driving ability of TFTs having a silicon film crystallized using catalyst elements alone and without laser irradiation. Alternatively, the resulting TFTs may exhibit big variation in properties although they have electric current-driving ability similar to that of those TFTs having a silicon film crystallized by laser irradiation alone. That is, even if the silicon film that has been crystallized by catalyst elements is further irradiated with laser light using the above conventional method as such, a further improvement was not achieved.
In the publication JP-A-7-221017 mentioned above, after a catalyst element is introduced into an amorphous silicon film, heat treatment is performed for a short period of time to form crystal nuclei only, and then laser irradiation is performed to crystallize the amorphous silicon film. That is, the main crystallization is performed by the irradiation with laser light, and this crystallizing method is suitable for sufficiently obtaining good results from the laser irradiation. However, it is still difficult to sufficiently control the formation of crystal nuclei. Therefore, it is difficult to use such a method in practical use.